https://screenrant.com/mash-show-blake-shock-death-groundbreaking/

M*A*S*H broke ground in many ways, but the shock death of Colonel Blake in season 3 did something no television show before it had attempted.

Lady Marjorie of Upstairs, Downstairs had gone down with the Titanic about a year and a half before. Had been seen on Masterpiece Theater in the US.

https://screenrant.com/mash-show-blake-shock-death-groundbreaking/

M*A*S*H broke ground in many ways, but the shock death of Colonel Blake in season 3 did something no television show before it had attempted.

In fact the statement "no television show before it had attempted" is not true. In1961 daytime drama The Edge of Night killed off the female lead character Sara Carr. The house door was accidently left open, the young daughter of Sara is seen going out the door with Sara racing after her, off camera Sara is struck by a car as she pushed her daughter out of the way of the oncoming vehicle. There's a big death scene played out at the hospital with her husband (the male lead) Mike Carr. As the story goes the CBS switchboard was inundated with phone calls to such an extent that the actress playing the part appeared before the start of the next episode to explain her decision to leave the series.

As far as primetime series go, The Danny Thomas Show had an offscreen death of the female lead character (this happened between seasons). By the time season 3 of MASH was completed the series had been dealing regularly with death and the horrors of war so the Blake offscreen death was not that shocking (the audience knew Stevenson was leaving the series, it had been reported in the press).